The present invention relates to the arts of pollution abatement by removal of sulfur and sulfur based compounds from the effluent of industrial processes and efficient conversion of sulfur and sulfur compounds in the effluent to sulfur dioxide prior to such removal.
It is well known that there is considerably interest in using forms of energy other than petroleum. One of the most plentiful sources of energy is coal. Conventional recovery of energy from coal is by combustion with the products of combustion discharged into the environment. Regrettably, a lot of coal has a high sulfur content. An end product of burning coal with sulfur is sulfur dioxide (SO.sub.2), relatively small quantities of which in the atmosphere being environmentally unacceptable. Elemental sulfur and other compounds of sulfur are environmentally deleterious and are produced in the tail gases of such processes as the Claus process.
Solutions are being sought for using high-sulfur coal in the production of energy without polluting the environment. One approach to the problem is to burn the coal in a conventional power plant and to remove the resulting sulfur dioxide pollutants from the stack gas discharge from the plant. Various stack gas treatment processes have been suggested, but none are very satisfactory. Some provide adequate treating efficiency but produce large quantitites of non-regeneratable waste. Others cannot treat the gas to sufficiently low sulfur dioxide residual levels to satisfy the requirements of the environment, and still others require the expenditure of too much energy.
Thus a process is needed that will remove the sulfur dioxide to acceptably low levels, that will recover the sulfur in an easy to handle, non-polluting form, and that will do so at an acceptable cost in terms of capital, operating, and energy requirements.
The conversion of the sulfur dioxide by reduction to sulfur by the Claus process is well known. This process reacts hydrogen sulfide and sulfur dioxide in the presence of a catalyst to form sulfur and water in accordance with the following reaction: ##STR1##
The Claus process is widely used in the petroleum refining industry but is not directly applicable to the removal of sulfur dioxide from stack gas. The limitation of the Claus process as conventionally practiced is that it is not capable of achieving sufficiently low sulfur dioxide concentrations. However, it is recognized that the chemistry of the Claus process can be used to obtain satisfactory sulfur dioxide removals if the temperature of the reaction is reduced. But at low temperature, the sulfur is deposited on the catalyst poisoning the catalyst.
In work done for the National Air Pollution Control Administration (NTIS Report No. PB 200 071) by Princeton Chemical Research Inc. it was shown that a low temperature Claus reaction could be used to satisfactorily treat stack gas from a power plant burning coal containing 3% sulfur. The greatest difficulty with their process is that it calls for the use of fixed beds of catalyst that must be periodically taken from service and heated to remove the sulfur. This regeneration step consumes too much energy. Furthermore, the catalyst is loaded with sulfur for a substantial portion of the on-stream cycle, the loading reducing the catalyst activity and process efficiency.
The products of reaction of the Claus process include more than sulfur and water. Residual amounts of sulfur dioxide and other environmentally sensitive sulfur compounds result. Accordingly, in tail gas treatment from a Claus plant it is desirable to remove all products of concern. Obviously, the economics of conversion are extremely important.
Today, the permissible amount of sulfur in stack gas is determined on the basis of the sulfur expressed as units of sulfur dioxide. Thus 32 grams of elemental sulfur are expressed as 64 grams of sulfur dioxide.